Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/1062—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon

Definitions

• the invention relates to a method and to a device for opera ⁇ ting a laser in an optical component.
• a passive optical network is a promising approach regarding fiber-to-the-home (FTTH) , fiber-to-the-business
• FTTB fiber-to-the-curb
• FTTC fiber-to-the-curb
• the PON has been standardized and it is currently being deployed by network service providers worldwide.
• Conventional PONs distribute downstream traffic from the optical line ter ⁇ minal (OLT) to optical network units (ONUs) in a broadcast manner while the ONUs send upstream data packets multiplexed in time to the OLT.
• OLT optical line ter ⁇ minal
• ONUs optical network units
• communication among the ONUs needs to be conveyed through the OLT involving electronic proces ⁇ sing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network.
• wavelength-division multiple- xing is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different sig ⁇ nals. This allows for a multiplication in capacity, in addi ⁇ tion to enabling bidirectional communications over one strand of fiber.
• WDM systems are divided into different wavelength patterns, conventional or coarse and dense WDM.
• WDM systems provide, e.g., up to 16 channels in the 3rd transmission window (C- band) of silica fibers around 1550 nm.
• Dense WDM uses the sa ⁇ me transmission window but with denser channel spacing.
• Channel plans vary, but a typical system may use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 25 GHz spacing.
• Amplification op ⁇ tions enable the extension of the usable wavelengths to the L-band, more or less doubling these numbers.
• Optical access networks e.g., a coherent Ultra-Dense Wave ⁇ length Division Multiplex (UDWDM) network, are deemed to be the future data access technology.
• UWDM coherent Ultra-Dense Wave ⁇ length Division Multiplex
• the respective wavelength is selected by the tuning of the local oscillator (LO) laser at the ONU.
• LO local oscillator
• the UDWDM system uses a tunable LO laser with a cohe ⁇ rent receiver to select one of a multitude of downstream wa- velengths.
• the correct wave ⁇ length is unknown and needs to be found by scanning for permissible wavelengths.
• a detection of downstream wavelengths from the OLT at the ONU site may be difficult. It can be done by processing a scan of the ONU ' s local oscillator over the whole wavelength range. As the width of the laser is rather small compared to the whole wavelength range and as the local oscillator laser and the downstream wavelength need to be nearly identical, such scan process requires a significant amount of time during which the local oscillator crosses the wavelength range in very small steps.
• Coherent receivers use narrowband lasers as local oscilla- tors.
• the permissible maximum linewidth of the laser depends on the permissible bit error rate (BER) of the communications system, the data rate, and the modulation format.
• BER bit error rate
• a maximum linewidth of the laser may amount to (less than) 1 MHz.
• the coherent receiver detects a DC signal that is proportio ⁇ nal to the total power of the local oscillator laser and the communication wavelengths impinging on the receiver together with an AC component.
• the DC component is independent of the frequency difference between the local oscillator (LO) la ⁇ ser's wavelength and the communication wavelengths and is filtered out before the data is processed any further.
• the AC component appears at a frequency which is the difference bet ⁇ ween the frequency of the LO laser and the frequency of the communication channel. The strength and the intensities of this AC component is proportional to a spectral overlap of the two lasers.
• the AC signal can be detected when the frequency difference is small enough to be within the bandwidth of the detection electronics and if the overlap persists long enough to make up a sizable fraction of the detection system's time
• both lasers may have a typical linewidth of, e.g., 1 MHz, and a typical com- munications band such as the C-band spans several THz
• a wa ⁇ velength scan would yield a signal only for a millionth of the possible scan positions, if the scan is performed in steps of the linewidth.
• the system would have to remain in each step long enough for the signal to register. Assuming that it takes a millisecond to register the overlap and stop the scanning, this would lead to a scan time of about 1000 s in order to find a channel. This time will even increase if information included in the channel (such as its availabili ⁇ ty) needs to be evaluated.
• the channel plan is sparsely used in order not to waste power on unused channels.
• most of the scan time would yield no signal at all, but since the scanning needs a certain time to detect the lack of a channel, unused spectrum will require nearly the same amount of scan time as used spectrum.
• the problem to be solved is to overcome the disadvantages described above and in particular to suggest a mechanism that allows speeding up a scanning process at an optical compo ⁇ nent .
• the laser is a local oscillator of the optical component
• the linewidth of the laser may be a
• the scanning process for a wavelength received at the optical component can be significantly accelerated as the laser's small linewidth does not have to scan a huge range of wavelengths for the in coming signal.
• an incoming signal is scanned utilizing the broadened linewidth of the laser.
• Such scanning of the incoming signal may in particular utilize determining a wavelength of the incoming signal.
• the laser's wavelength is preferably adjusted to such wavelength of the incoming signal.
• the laser is locked onto the incoming signal.
• the linewidth may again be decreased (e.g., no longer be broadened) and the laser of the optical component can be lo ⁇ cked onto the incoming signal, i.e. the wavelength of the la ⁇ ser is chosen (or adjusted) such that it locks onto the wave ⁇ length of the incoming signal.
• the (previously) broadened linewidth of the laser can be used to determine an incoming signal in an accelerated manner and to lock onto such signal.
• the incoming signal may be a signal received from an OLT and the optical component may be an ONU.
• the scanning speed is reduced before the laser is locked onto the incoming signal.
• the scanning speed can be reduced, in particular together with the decrease of the li ⁇ newidth of the laser.
• a wavelength of the laser is fine-tuned before the laser is locked onto the incoming signal.
• Such fine-tuning or fine-adjustment is an optional step to adjust the wavelength of the laser to correspond to the wave- length of the incoming signal. It depends on the respective laser and scenario whether or not such step of fine-tuning may be favorable.
• the linewidth of the laser is broadened by swiftly modulating a bias current of the laser.
• the linewidth of the laser is broadened by mechanical modulation of a component of the la ⁇ ser .
• the component of the laser is a mirror of the laser, wherein such mirror can be modulated via a piezoelectric actuator or a micro electromechanical system.
• the component of the laser is a mechanical tuning element, in particular a dielectric filter of the laser.
• the linewidth of the laser is broadened by increasing the laser's current such that the la ⁇ ser enters a multimode. According to a next embodiment, the linewidth of the laser is broadened by not compensating vibrations.
• vibrations may not be compensated thereby broadening the linewidth of the laser.
• the linewidth of the laser is broadened by setting the laser's current below a determined threshold such that the laser serves as broadband light sour ⁇ ce and wherein a tunable external cavity of the laser is mo- dified in particular by a dielectric filter.
• such determined threshold may be set pursu ⁇ ant to a characteristic curve determined for the particular laser .
• the optical component is an optical network unit or an optical line terminal.
• the optical component may be an op- tical network unit (ONU) connected towards an OLT .
• the OLT provides a downstream signal that is to be detected by the ONU during the scanning process as described.
• a processing unit that is arranged - for broadening a linewidth of the local oscillator laser in particular during a scanning process.
• the optical element may be an ONU or an OL .
• optical com- munication system comprising at least one optical element as described herein.
• Fig.l shows an arrangement comprising a local oscillator laser, splitters, a modulator and a receiver, wherein such components could be part of an ONU;
• Fig.2 shows a schematic scenario with an OLT connected via a wavelength filter towards several ONUs;
• Fig.3 shows a diagram depicting the basic principle of a filter which determines the gain profile of a local oscillator signal
• Fig.4 shows an alternative embodiment as how an incoming and an outgoing signal may be influenced by a single tunable element, e.g., a filter;
• Fig.5 shows a schematic block diagram of an optimized scan ⁇ ning process to adjust a wavelength of a LO laser to an OLT signal
• Fig.6 shows a schematic diagram visualizing several options as how to broaden the spectrum of the LO laser during scanning .
• the solution provided solves the problem of a slow scanning speed in particular by spectrally broadening the local oscil ⁇ lator laser during the scan process. While no signal is detected, the linewidth of the laser is broadened in particular to a maximum value feasible. This significantly accelerates the scan speed.
• the linewidth of the laser is decreased and the scan speed is decreased so that the lo ⁇ cal oscillator (LO) laser's linewidth may reach the procedure of the usual scan with small linewidth.
• the LO laser approaches the actual channel in a much faster manner compared to scanning the spectrum with the linewidth of the LO laser's wavelength.
• Fig.5 comprises a schematic block diagram visualizing the aforementioned steps.
• a step 501 the LO laser spectrum (linewidth) is broadened during the scanning process.
• the broadened LO laser signal is used for scanning for an OLT signal. In case the signal is detected (see step 503), it is branched to a step 504. Otherwise, the scanning of step 502 is continued.
• the spectrum (li ⁇ newidth) of the LO laser is decreased and the scanning speed is reduced.
• a fine adjustment of the LO laser wavelength can be conducted to lock onto the wavelength of the downstream OLT signal.
• the frequency of the LO laser is modulated by swiftly modulating the laser bias current (see block 601) .
• This is applicable in particular for all solid state lasers and bears the advantage that existing electronic compo ⁇ nents (laser driver) can be utilized; hence, no addi ⁇ tional parts are required.
• the broadening of the laser's spectrum can be achieved by mechanical modulation (see block 602) .
• the length of the laser can be modulated by modulating a laser's mirror that is used on extended cavity lasers. This could be achieved via a piezoelectric actuator or a micro electromechanical system (MEMS) attached to one of the cavity mirrors.
• MEMS micro electromechanical system
• This concept allows for large increases of the linewidth and is in particular useful in case tuning of the laser and is already achieved by such a mechanical actuator.
• the broadening of the laser's spectrum can be achieved by oscillations of a mechanical tuning element such as a dielectric filter (see block 603) .
• a mechanical tuning element such as a dielectric filter
• this method does not require additional parts to be provided. However, it may be useful to provide a fast moving jig ⁇ gle unit to efficiently broaden the laser's spectrum.
• the laser's current can be increased such that a laser diode of the laser enters multimode (see block 604) .
• This approach also bears the advantage of increasing the power of the broadened signal.
• the laser's linewidth can be broadened by not compensat ⁇ ing vibrations during the scan process (see block 605) .
• the accelerated scanning procedure may comprise two parts :
• a characteristic curve an op ⁇ tical output power in view of a driving current
• the measurement device e.g., a power detection diode
• the laser current can be set by soft ⁇ ware so that the measurement of the characteristic curve only requires some piece of software and no additional hardware. Also, this measurement of the characteristic curve can be performed quickly.
• a threshold current for the laser can be determined.
• This threshold current information is used for an accelerated scan:
• the laser current is set a little (e.g., some percent) below this threshold current.
• the laser hence serves as a broadband light source, but its spectrum is modified by a tunable external cavity (see block 606), which is implemented by the dielectric filter and the tuning angle of such di ⁇ electric filter (see Fig.3 and Fig.4 and descrip ⁇ tions thereof below) .
• the laser delivers a narrow optical spectrum, but no lasing.
• This narrow optical spectrum is spectrally signifi ⁇ cantly broader than the laser line and can be used for a quick scan. If an OLT wavelength is detected by an increase of the received intensity, the ONU laser current is increased above threshold and the detailed scan with reduced linewidth of the laser may start.
• output power of the sub-threshold laser in ⁇ creased by a post amplifier may be of advantage.
• the need for such post amplifier depends on the system's characteristics.
• such am ⁇ plifier may be included to provide sufficient opti ⁇ cal power to the subsequent optical stages; thus the concept suggested may efficiently utilize this amplifier without any need for additional hardware.
• a large wavelength range can be scanned quickly without additional hardware ef ⁇ fort .
• this approach detects any OLT wavelength in a reliable way. Without broadening the spectrum an OLT wavelength may be overlooked because of mode discontinuity of the ONU laser during a detailed unidirectio ⁇ nal scan. If the ONU knows that an OLT wavelength is avai ⁇ lable in a certain wavelength window, it can take additional measures in case the regular detail scan does not find it.
• Fig.l shows an arrangement comprising a local oscillator la ⁇ ser 101, splitters 103, 105 and 106, a modulator 104 and a receiver 102. These components may be part of an ONU 111.
• An optical fiber 108 may be connected towards an OLT (not shown) .
• the signal generated at the local oscillator laser 101 is mo ⁇ dulated via the modulator 104 to produce an upstream data signal 109 to be conveyed via the optical fiber 108.
• An inco- ming optical signal via fiber 108 is fed to the receiver 102.
• the signal generated at the local oscillator laser 101 is fed via splitters 103 and 105 to the receiver 102.
• the local oscillator laser 101 is used for modulation purpo- ses to transmit the signal from the ONU 111 to the OLT and for reception purposes regarding the incoming received signal 110.
• the wavelength of the local os ⁇ cillator laser 101 needs to be adjusted to the wavelength of the incoming signal.
• the approach described herein allows for an accelerated scanning process in order to detect the lock onto the incoming signal within a short period of time.
• Fig.2 shows a schematic scenario with an OLT 201 connected via a filter 202 (e.g., a wavelength filter or an AWG) to- wards several ONUs 203, 204.
• a direction from the OLT 201 to ⁇ wards the ONU 203, 204 is referred to as a downlink or downstream direction, whereas the opposite direction from the ONU 203, 204 towards the OLT 201 is referred to as uplink or upstream direction.
• Fig.3 shows an optical gain element 306 (e.g., a laser active medium) providing an optical signal via a waveguide 305 and a lens 304 to a filter 303, which is in particular an angle- tunable dielectric filter.
• the combination of said filter 303 and the optical gain element 306 comprising a mirror 307a constitutes a tunable laser 308, i.e. the tunable laser 308 can be adjusted via said filter 303.
• Another mirror 307b is located between the filter 303 and the output of the tunable laser 308.
• the output of the tunable laser 308 is conveyed to a splitter 312, which further directs it to a modulator 311 and to another splitter 309.
• the signal from the local oscillator 308 is modulated by the modulator 311 and fed to a circulator 302 that conveys the modulated signal via a waveguide 301.
• a data signal is conveyed via the wavegui ⁇ de 301 to the circulator 302, which feeds the data signal to ⁇ wards the splitter 309 where it is combined with the signal from the tunable laser 308.
• This combined signal is directed to a local receiver 310 for further processing purposes.
• FIG.3 can be provided with an opti ⁇ cal network component, e.g., with an OLT or an ONU.
• Fig.4 shows an exemplary arrangement as how an incoming and an outgoing signal may be influenced by a single tunable ele- ment, e.g., a filter.
• An optical gain element 401 comprises a Semiconductor Optical Amplifier (SOA) 405 via which a signal is being conveyed to ⁇ wards a filter 403 and reflected by a mirror (or reflector) 404 back to the optical gain element 401.
• SOA Semiconductor Optical Amplifier
• the filter 403 can be used to adjust a wavelength of a laser.
• the opti ⁇ cal gain element 401 comprises a mirror 402 that is used to reflect the incoming signal from the filter 403 to be modula ⁇ ted by a modulator 409 and provided as an output signal "Sig- nal Out" 407.
• the optical gain element may be a laser dio ⁇ de comprising an anti-reflection coating.
• an input signal (data signal, "Signal In” 410) can be fed via the filter 403 to a photodiode 408.

Applications Claiming Priority (1)

Publications (2)

Family

ID=42169421

Family Applications (1)

Country Status (4)

Families Citing this family (4)

Family Cites Families (5)

• 2009
  • 2009-09-11 CN CN200980162416.0A patent/CN102598547B/zh active Active
  • 2009-09-11 EP EP09782911.3A patent/EP2476213B1/de active Active
  • 2009-09-11 WO PCT/EP2009/061800 patent/WO2011029478A1/en active Application Filing
  • 2009-09-11 US US13/395,529 patent/US8824034B2/en active Active

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events